Field of the Invention
The invention relates generally to the field of electric motors. In one aspect, the present invention relates to sensorless permanent magnet synchronous motors (PMSM) and application safety.
Description of the Related Art
Permanent Magnet Synchronous Motors (PMSM) or Brushless DC (BLDC) motors are increasingly popular electric motor design which replaces wear-prone brushed DC motors with an electronic controller that improves the reliability of the unit and reduces the footprint and size, making such PMSM or BLDC motors suitable for applications where space is tight. However, the absence of mechanical or electrical contact between the stator and rotor of the PMSM or BLDC motors presents challenges with determining the relative positions of the component parts. One position-indicating arrangement is to use sensors (e.g., incremental encoders) to detect a motor component position status as part of the motor control function, but such sensors add cost and increase complexity (due the additional sensors, wiring, connectors, soldering, etc.), reduce reliability (due in part to the sensor connectors that are prone to contamination from dirt and humidity), and are not suitable for applications (e.g., where the rotor is in closed housing and the number of electrical entries must be kept to a minimum, such as in a compressor, or in applications where the motor is immersed in a liquid such as some pumps). To address such shortcomings, sensorless motor control techniques may be used to detect a motor component position status. For example, one position-indicating arrangement is to detect the potential or electromotive force (EMF) generated in the windings which gives rise to secondary magnetic field that opposes the original change in magnetic flux driving the motor's rotation. In resisting the motor's natural movement, the EMF is referred to as a “back” EMF. However, sensorless motor control techniques have a drawback that the rotor mechanical block or stall condition may not be recognized by the sensorless algorithm. The inability to detect a locked or stopped rotor presents potential application safety concerns which are increasingly required for household motor control application standards, such as IEC 60730 (“Automatic electrical controls for household and similar use”) or IEC 60335-1 (“House and Similar Electrical appliances”). As a result, the existing solutions for detecting locked rotor conditions are extremely difficult at a practical level.